Hearing aid systems with two hearing aid devices which can be worn on the head for binaural provision of a user are known from the prior art. Furthermore communication systems are known in which different acoustic signals can be directed to a user via at least two loudspeakers for the left ear and the right ear.
A sound output device for a motor vehicle is known from DE 103 03 441 A1. An output section consisting of a pair of loudspeakers which are arranged adjacent to one another, is installed in a seat backrest or in the back of a designated seat. The sound output surfaces of the loudspeakers point in each case towards the designated person who is sitting on the designated seat. This makes it easy to ensure that there is the distance required avail able to achieve a clear acoustic image localization in keeping with the size of loudspeakers, which work together to form the output section.
A hearing device that can be worn on the head is known from EP 0 557 847 B1, said device comprising an electrical signal path between a microphone and an earpiece, with the signal path being able to be adapted by using means to electronically adjust pre-programmable transmission parameters and a switching means of the hearing device to different hearing situations/sound environments, with the switching means additionally controlling a signal output device which emits at least one signal which is characteristic for the transmission parameters set for a specific hearing situation/sound environment, with the hearing device user being able to perceive this signal and being able to be informed about the selected setting without removing the hearing device from their head.
To determine the sound pressure which any given signal produces in front of a person's eardrum it is sufficient to know the impulse response between the source and the eardrum. This is referred to as the HRIR (Head Related Impulse Response). Its Fourier-transformed function is called the HRTF (Head Related Transfer Function). The HRTF comprises all physical characteristic values for localization of a signal source. If the HRTFs are known for the left and right ear this enables binaural signals of an acoustic source to be synthesized.
In a non-resonating environment the HRTF is a function of four variables: The three space coordinates (in relation to the head) and the frequency. For determining the HRTFs, measurements are mostly performed on an artificial head, e.g. KEMAR (Knowles Electronics Mannequin for Acoustical Research), A known overview of how HRTFs are determined can be found for example in Yang, Wonyoung, “Overview of the Head-Related Transfer Functions (HRTFs)”, ACS 498B Audio Engineering, The Pennsylvania State University, July 2001.
It is known from the area of artificial head technology that direction-dependent transmission functions of the head and the outer ear can be simulated relatively precisely by multiple microphone arrangements in the free field with suitable downstream filters (e.g. Podlaszewski, Mellert: “Lokalisationsversuche for virtuelle Realität mit einer 6-Mikrofonanordnung” (Localization trials for virtual reality with a 6-microphone arrangement), DAGA 2001). The filters are designed here with a special optimization procedure so that the sum of the filtered microphone signals (typically 3 per side) for any given spatial directions, corresponds with a certain error tolerance to the sound signal which would be measured in the same situation at an artificial head.